1. Field of the Invention
The present invention relates generally to the field of integrated circuits. More particularly, the present invention relates to the field of charge pumps.
2. Discussion of Related Art
Charge pumps are circuits that raise the voltage of a voltage signal. Typically, charge pumps raise the voltage signal to a higher voltage than the signal generated from the power supply of the integrated circuit ("IC"). For example, programmable logic devices may require a very high voltage (i.e. between 7-12 volts) to program the IC although the IC operates with a 5 volt power supply. Therefore, a charge pump is used to raise the 5 volt signal generated by the power supply to 7-12 volts during programming.
Although many different charge pump architectures exist today, the efficiency of the charge pump is typically related to how quickly the charge pump charges up the voltage signal. Current charge pumps use capacitive devices to charge the voltage signal. If the capacitive device is implemented with a metal oxide-semiconductor ("MOS") transistor, then the capacitive device does not start charging until the voltage across the gate terminal and the source/drain terminal approaches the threshold voltage of the MOS transistor.
As new technologies are developed to increase the speed of an integrated circuit, many integrated circuits are designed to operate with smaller power supplies such as a 3.3 volt power supply. By reducing the switching time between a logic "high" voltage level and a logic "low" voltage level or vice-versa, the IC is capable of operating at a faster speed. However, as the switching time is reduced, the lag period for turning "on" a capacitive device becomes a larger portion of the switching time. Therefore, it is desirable to turn "on" the capacitive device as soon as a voltage is applied to its input, especially in a high speed IC. This can be achieved by permanently turning "on" the MOS transistor. By eliminating this lag period, the charge pump raises the voltage level of a voltage signal more efficiently.
Because faster ICs generally consume more power than comparatively slow circuits, the circuit designer is challenged to minimize the overall power consumption of a high speed charge pump. Typically, the ring oscillator in a charge pump consumes a significant portion of the power consumed in a charge pump. Reducing the power consumption of the ring oscillator can be achieved by reducing the power consumption of each inverter in the ring oscillator. If the ring oscillator is implemented with CMOS inverters having a p-channel MOS transistor coupled to an n-channel MOS transistor, then the power consumption of the inverters can be reduced by eliminating its short circuit or crow-bar current. The short circuit or crow-bar current refers to the current that flows between Vcc and ground during the temporary period in which both the transistors are on (i.e. when the inverter switches voltage levels). Therefore, it is desirable to reduce the power consumption of the ring oscillator in a charge pump by eliminating short circuit or crow-bar current.